In recent years, light emission efficiency of light-emitting diodes (LEDs) has improved to such an extent that LEDs are attracting attention as a light source for general illumination use. In order to drive LEDs, a DC power supply is required. As a conventional DC power supply, a power supply circuit having a voltage step-down circuit has been proposed (Patent Literature 1). The voltage step-down circuit reduces the voltage supplied from an AC power supply by rectification and smoothing using a switching element, and outputs the voltage thus reduced.
In general, such a power supply circuit performs control for suppressing the current flowing through the LEDs such that the current does not exceed a predetermined current value, thereby enabling the LEDs to emit light in a stable state. When such control is required, the power supply circuit is generally provided with a current detection resistor for detecting the current flowing through the LEDs and a fixed-voltage supply using a Zener diode, and on-off operations of the switching element are controlled based on the difference between the voltage between the terminals of the current detection resistor and the output voltage from the fixed-voltage supply. The current flowing through the LEDs are thus controlled such that the current does not exceed a predetermined current value.